PHPUnit with --group

I'm starting to work with PHPUnit for unit testing my code. I'm just using the standard boostrap file and UnitTestCase from he docs.
I'm trying to group my tests by e.g. @group models, and the run phpunit --group models. The test runs fine and i get the reults, but I'm also getting an nncaught exception 'PHPUnit_FrameworkIncompleteTestError' with message 'Please run parent::setUp(). This is from the \_destruct() method in UnitTestCase. The stange thing is that this exception is not thrown if i just run phpunit. Can anyone explain this?